Many varieties of solid state lasers are known of which the solid state laser medium is pumped by laser diodes (DPSSL: Diode Pumped Solid State Laser).
The instant invention is concerned, among others, with the problem of how to adjust the energy of laser beam pulses as emitted by a diode pumped solid state laser.
A quality switch (Q-switch)--also referred to as Q-control--is understood as being a method of generating short, high peak power laser pulses by interrupting the beam path in the optical cavity resonator by means of an optical switch (Q-switch) so as to prevent the laser from starting until the population inversion caused by the pumping process lies way above the value determined by the threshold condition. Only then does the Q-switch clear the beam path, in other words it controls the quality of the optical resonator so that the laser can start, and the energy stored in the active laser medium will be emitted in the burst of a very short, intensive laser radiation pulse. Pulse lengths in the range of nano seconds are quite typical.
Known methods of adjusting the pulse energy of Q-switched solid state lasers operate by adjustment of the desired output energy of the laser pulses on the basis of the pumping energy introduced into the laser medium, respectively the amount of the energy used by the Q-switch pulse. The known methods of adjusting the pulse energy may be classified in three groups:
(1) The intensity of the pump radiation and thus the energy stored in the laser medium is varied, at constant duration or length of the pumping pulse.
(2) The duration or length of the pumping pulse and thus the energy stored in the laser medium is varied, at constant intensity of the pump radiation.
(3) The intensity and the duration or length of the pumping pulse remain unchanged; the energy of the laser pulse emitted is controlled by opening the Q-switch at a time when the energy stored in the laser medium has reached a certain value.
These known methods give rise to technical problems. In the case of the methods listed sub (1) and (2) above, the mean optical power introduced into the laser medium varies and, therefore, also the thermal loading of the solid state laser medium varies and, as a consequence thereof, the so-called thermally induced lens and the thermally induced double refraction undergo changes. With methods (1) and (2) it is especially the thermally induced lens in the laser medium which causes difficulty because normally the laser resonator is optimized, being tuned precisely to a certain pumping energy or average pumping power and, therefore, being adjusted to a very specific thermal lens (variation of the pumping energy consequently may lead to mal-adjustment and loss of optimization).
In the case of method (3) it is not the thermally induced lens which is causing the problem because the mean energy introduced into the laser medium remains constant, and only warming up of the laser medium by the laser beam itself can influence the characteristics of the thermally induced lens. Due to the very low absorption of the laser radiation in the laser medium, however, these effects usually are minor (yet measurable) with the method described sub (3). It is a disadvantage of this third method that the length in time of the laser pulses is highly dependent on the adjusted pulse energy. The laser pulse duration, among others, is a function of the amplification in the laser medium and that depends on the energy introduced into the laser medium up to the moment the Q-switch (e.g. Pockels' cell) switches. This is a difficulty which basically occurs with methods (1) and (2) as well. With them, however, the principal disadvantage is the effect brought about by the thermally induced lens in the laser medium. Varying the pulse length while varying the pulse energy is undesirable especially for certain applications of the laser, such as optically non-linear processes (producing higher harmonics of the laser radiation, optical parametric processes) or where laser radiation is applied in time-dependent examinations.
The modulation of light for purposes of data transmission is described in a paper by G. Grau, K. Gurs, R. Muller, and D. Rosenberger entitled "Modulation von Licht mittels elektrisch doppelbrechender Kristalle", published in the journal Zeitschrift fur angewandte Physik, 1964, vol. 17. no. 1, pages 16 to 20. Information to be transmitted is impressed upon a laser pulse while the pulse endures. This prior art does not deal with adjusting the pulse energy. Instead, its interest is focussed on how to achieve quick switch-on and switch-off of the light beam.
U.S. Pat. No. 5,197,074 describes a modulator in a laser cavity resonator by means of which the resonant quality is adjustable for the purpose of adjusting the pulse energy of the laser pulses emitted.